I just finished freshly installing Arch on a new Pi3 B+. It booted up on ethernet fine, but this pi is intended for a place where it will need wifi. I noticed wlan0 was not in 'ip addr' list. A quick check of dmesg showed the following error on boot:
$this->bbcode_second_pass_code('', 'brcmfmac mmc1:0001:1: Direct firmware load for brcm/brcmfmac43455-sdio.txt failed with error -2
')
Hunting around the interwebs (including this forum) turned up no obvious answers, although eventually the breadcrumbs led to the most obvious solution: find brcmfmac43455-sdio.txt and stick it somewhere the driver could find it. Fortunately I had a Pi3 running Raspbian, so I went hunting for the file there and found it in /lib/firmware/brcm. I copied it over to the same place on my fresh Arch install, reloaded the module and viola! Good to go.
Posting here so the next person to run into this error will hopefully find this post from Google and have a quick fix handy. Even better if one of the Arch devs can add this text file into the image for the Raspberry Pi so new users don't get this error at all.